The present invention relates to a finger joint prosthesis, more particularly to a finger joint prosthesis preferable for being used for a metacarpophalangeal joint or a proximal interphalangeal joint or the like.
A metacarpophalangeal joint (hereinafter, referred to as MP joint) is constituted by a metacarpal bone and a proximal phalanx and a proximal interphalangeal joint (referred to as PIP joint) is constituted by a proximal phalanx and a middle phalanx. When a patient suffers from arthritis represented by rheumatoid arthritis, these joints, ligaments and tendons are gradually affected and deformation with ache and significant limitation in motion are shown.
As a countermeasure therefor, an operation is carried out for replacing a joint portion deformed by arthritis by a finger joint prosthesis.
Conventionally, such a finger joint prosthesis is generally provided with a structure having a middle flange portion and a shaft portion projected from the flange portion as a base end in the axial direction by using an elastic polymer material represented by silicone. However, according to the structure, the material is excessively flexible and therefore, wear or break is liable to result and further, the structure is weak at twist and therefore, nonphysiologic side flail behavior or circumflexing is liable to result.
Although a finger joint prosthesis made of a metal using a hinge has been proposed as a countermeasure therefor, it is the actual state that a practical product has not been proposed yet since a movable range is limited or physiologic movement is not carried out.
It is an object of the present invention to provide a finger joint prosthesis which is well matched with the motional characteristic of the finger and is highly practical with a comparatively simple structure.
A finger joint prosthesis according to the present invention is not only preferable to a metacarpophalangeal joint (hereinafter, referred to as MP joint) and proximal interphalangeal joint (hereinafter, referred to as PIP joint) of hand or foot as well as applicable to a distal interphalangeal joint (DIP joint) and further is effective as a joint prosthesis for hand or elbow other than the finger joint.
In order to achieve the above-described object, the present invention is constituted as follows.
According to an aspect of the present invention, there is provided a finger joint prosthesis of a style where stems are embedded into tubular bones at a remotely disposed portion and a nearly disposed portion of a finger joint, said finger joint prosthesis comprising a socket having an opening at one portion thereof and a spherical face at an inner side thereof and integrally formed with a stem on a side opposed to the opening, a head having a spherical portion to be fitted to the spherical face and integrally formed with a stem extending outwardly from the opening, a sheath surrounding either of the stem of the socket and the stem of the head, and wherein the stem can be moved slidably in an axial direction by adhering and fixing the sheath to a narrow cavity at the nearly disposed portion or the remotely disposed portion.
As one embodiment of the present invention, the spherical face is provided with a groove reaching the opening in a direction in which the stem is to be flexed and the spherical portion of the head is formed with a projected streak fitted to the groove. In this case, when the width of the groove is made larger than the width of the projected streak thereby intentionally providing play (gap) therebetween, pertinent side flexing or circumflexing can be carried out and accordingly, it is preferable to an MP joint. When the play is not provided, it is preferable to a PIP joint restricting the side flexing.
As other embodiment, the socket is formed with an opening in a shape of a long hole and a spherical portion of the head is fitted via the opening. Further, a projected trapezoidal portion having a conical side wall face is provided at the spherical portion of the head and the stem is extended forwardly from the projected trapezoidal portion. The opening in a shape of a long hole is provided with an inclined face at its edge portion and the side flexing angle is restricted by bringing the conical side wall face into contact with the inclined face. When the conical side wall face is formed into a pyramidal shape, both of the side flexing angle and circumflexing angle can be restricted by a simple structure. Further, when the conical side wall face and the inclined face are brought into a distance relationship where they are brought into contact to each other or proximate to each other, the side flexing is hampered and therefore, a preferable PIP joint can be constituted.
Further, although the socket and the spherical portion of the head may simply be fitted to each other, a pin may be penetrated through the spherical portion to be orthogonal to the direction of inclination of the stem. Thereby, the spherical portion of the head can firmly be prevented from coming off. Also, when window holes sufficiently larger than the outer diameter of the pin are provided in the socket, the circumflexing can simultaneously be restricted.
The above-described sheath not only promotes the adhesive performance in respect of a bone but guarantees very small extension in the axial direction accompanied by double axes type flexing motion particular to a joint. In addition thereto, the surface pressure between the socket and the spherical portion in the flexing and extending motion is reduced and smooth flexing and extending motion can be carried out over a long period of time. Preferably, the sheath is made of a metal material excellent in wear resistance and corrosion resistance.
According to the present invention, not only the flexing and extending motion can be carried out by a comparatively simple structure but both of the side flexing and circumflexing motion can be carried out by the ball joint structure and further, since the sheath is embedded and adhered into a narrow cavity and the sheath on the side of the socket or the sheath on the side of the head is slidably fitted thereto in the axial direction, smooth escape is provided, excessive surface pressure is not applied between the socket and the head in the flexing and extending motion by which the flexing motion can be carried out smoothly. Further, wear or destruction can be prevented and accordingly, the motional function that is stable over a long period of time can be achieved.
When play is intentionally provided between the groove on the equator of the spherical face of the socket and the projected streak of the spherical portion of the head, pertinent side flexing and circumflexing within a constant limit can be carried out by the play and a joint suitable for an MP joint can be provided.
When a groove both ends of which communicate with the opening portion is provided on the equator of the spherical face and the projected streak having a width substantially in coincidence with the width of the groove is formed on the outer periphery of the spherical portion of the head, smooth and stable flexing and extending function can be achieved and further, the side flexing and the circumflexing can be hampered by the groove on the equator of the spherical face of the socket and the projected streak of the spherical portion of the head and accordingly, an excellent PIP joint can be provided.
When a conical side wall face is provided at the spherical portion of the head and the side flexing angle is restricted by bringing the conical side wall face in contact with widening inclined faces at edge portions of the opening of the socket, a simple and inexpensive structure can be constituted while having effective function as an MP joint.
When a pyramidal side wall face is provided at the spherical portion of the head and the side flexing angle is restricted by bringing the pyramidal side wall face in contact with a pair of inclined faces provided at an opening having a square contour on the side of the socket and the circumflexing angle is restricted by bringing corner portions of the pyramidal side wall face in contact with the pair of inclined faces, a simple and inexpensive structure can be constituted while having sufficient function as an MP finger joint prosthesis.
When a pin penetrating through the spherical portion of the head is provided and projected portions at both ends of the pin are loosely fitted to widow holes of the socket, even when violent pull is effected, the separation of the socket from the head can be prevented and further, an effect where the side flexing angle can be restricted to a constant angle further firmly and the circumflexing angle can be restricted constant by bringing the projected portions of the pin in contact with the window holes is achieved.
When the opening is constituted in a shape of a long hole having inclined faces, a pin is penetrated through the spherical portion of the head orthogonal to the axial line of the stem, holes for supporting the both end portions of the pin are provided to the socket, the stem is formed to the spherical portion of the head via a projected trapezoidal portion, a conical side wall face is provided at the projected trapezoidal portion and the conical side wall face and the inclined face are brought into a distance relationship where they are brought into the contact with each other or proximate to each other, the separation of the socket from the head can be prevented even with violent pull and the side flexing and the circumflexing can be prevented by the pin and the holes and accordingly, a PIP finger joint prosthesis having excellent function can be constituted.
When the head and the sheath are made of a metal material excellent in corrosion resistance and wear resistance represented by a titanium alloy or a cobalt chromium alloy and the socket is made of a polymer compound excellent in wear resistance represented by ultra high molecular weight polyethylene, a finger joint prosthesis having high durability can be constituted.
Although other features or advantages of the present invention will be clarified by a detailed description as follows, it is apparent that so far as basic feature of the present invention is provided, the present invention is not limited to constitutions shown by embodiments but a skilled person can carry out various changes and modifications without deviating from the scope of the present invention.